1. Field of the Invention
This invention relates to conveyors and more particularly, to conveyors for moving a succession of articles such as glass sheets through a treatment apparatus at multiple speeds. The invention has particular but not exclusive utility in the conveyance of glass sheets through a heat-treating apparatus.
When glass sheets are conveyed through a heat-treating apparatus, it is frequently desirable to impart different velocities successively to each glass sheet as the latter passes through successive treatment stations or transfers from one treatment station to another along the length of the apparatus. In a mass production operation, articles are frequently moved at a non-uniform velocity so that each article in turn moves at a different speed through one or more selected treatment stations or transfers between treatment stations than it does throughout the general length of the apparatus. In treating glass sheets, it is especially advantageous to be able to move them relatively slowly during their passage through certain treatment stations and more rapidly between other treatment stations.
In apparatus containing a roller-type conveyor for heat-strengthening or tempering glass sheets or for shaping glass sheets by roll forming followed by rapid cooling, it is customary to move a series of glass sheets at a relatively slow speed through a heating furnace and then move each sheet more rapidly through the roll forming station and cooling station. Unless the rolls which convey each glass sheet change speed according to a desired cycle, each glass sheet may be engaged simultaneously by conveyor rolls rotating at different speeds. Such simultaneous engagement causes roll marking and may cause sheet misalignment.
2. Description of the Prior Art
It is well known to use a roller type of conveyor in which articles to be conveyed are engaged directly by rotating rolls which drive the glass sheet by frictional engagement with their bottom surfaces. Successive conveyor rolls or sets of rolls may be rotated at different speeds as required to vary the velocity imparted to the glass sheet. However, when different portions of a glass sheet are simultaneously engaged by two different conveyor rolls rotating at different speeds, and the temperature of the glass is sufficiently high, the bottom surface of the glass becomes marked if the rotating velocity of different conveyor rolls engaging a glass sheet simultaneously differs significantly. Also, if the glass sheet is not of rectangular outline and/or its longitudinal axis through its center of gravity is not parallel to the path of the conveyor, the glass sheet tends to skew when it is simultaneously engaged by conveyor rolls rotating at different speeds.
U.S. Pat. No. 1,638,769 to Hitchcock discloses a system of conveyor rolls arranged so that a glass batch formed by casting and rolling is transferred to an annealing lehr at an initial high speed and then passed through the annealing lehr at a considerably lower speed so as to reduce the length needed for the annealing lehr. The arrangement in the Hitchcock apparatus is such that the glass passes from a high speed entry section of the lehr to a lower speed main section in such a manner that the glass is always supported by a set of rolls, all of which rotate simultaneously at the same peripheral speed. This uniformity in peripheral speed, first at a high speed and then at a lower speed, avoids the tendency to buckle the sheet which might occur if the sheet were engaged at its forward end by rolls rotating at a relatively slow speed while its trailing end is still being supported by rolls rotating at a relatively high speed. In the Hitchcock patent, the conveyor rolls are so arranged and timed that the forward end of a newly formed sheet conveyed downstream by rotation of the conveyor rolls at high speed approaches closely to the rear end of the preceding sheet which has been transferred to conveyor rolls rotating at a low speed before the rate of the high speed rolls is reduced to allow the use of the annealing lehr to its full capacity.
Another patent which discloses roller conveyors for treating glass sheets wherein one set of rolls is moved at a higher speed than the remaining rolls in the conveyor is U.S. Pat. No. 2,848,845 to Doumont. This patent provides high speed rolls at the exit of an annealing lehr to separate a broken section of a continuous glass ribbon from the remainder of the ribbon whenever the glass ribbon breaks during annealing. Separating avoids further glass breakage resulting from repeated collison of the leading edge of the newly formed continuous ribbon against the trailing edge of the broken ribbon portion.
Other patents that provide multiple speed roller conveyor systems for conveying glass sheets through a succession of glass treatment stations include Canadian Pat. No. 680,042 to Carson and the following U.S. Pat. Nos.: Ritter U.S. Pat. No. 3,338,695; Rahrig, O'Connell and Ferguson U.S. Pat. No. 3,372,016; Carson, Ferguson, Ritter and Hymore U.S. Pat. No. 3,396,000; Carson and Ritter U.S. Pat. No. 3,522,029 and Ritter U.S. Pat. No. 3,672,861.
Another patent that conveys glass sheets at a non-uniform velocity through a heat-treatment apparatus is U.S. Pat. No. 3,594,149 to Pickavance and Freestone. However, in this patent the glass sheets are continuously engaged along their edges by fingers which are caused to move at different speeds when moving the glass through or between different stations of the glass treatment apparatus. This latter patent does not move the glass sheet from roll to roll to provide sequential contact of different glass sheet portions as in a roller conveyor. Therefore, the glass edge engaging fingers are likely to distort the continuously engaged glass sheet edge portions.
Despite the patents enumerated, there still remains a need in the glass treatment art for heat-treatment apparatus provided with conveyors capable of ready adjustment of the rotational speed of rotating conveyor rolls that is also capable of rapid adjustment of conveyor speed to enable the conveyor to be adjusted rapidly to accommodate for conveying sheets of different lengths through a conveyor of minimum length and, at the same time, accomplish the objectives of first orienting and aligning the glass sheets properly relative to said conveyor and then avoiding roll marking and skewing of the glass sheets moved along said conveyor during a multiple step treatment at multiple speeds.